This invention relates to a method of etching apertures in a thin metal sheet to form a shadow mask for a color picture tube, and particularly to such a method that utilizes a magnetic assembly during etching of a thin tension shadow mask, to magnetically hold the mask material.
A color picture tube includes an electron gun for generating and directing three electron beams to the screen of the tube. The screen is located on the inner surface of a faceplate of the tube and is made up of an array of elements of three different color emitting phosphors. A color selection electrode, or shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel or Invar, that is usually contoured to somewhat parallel the inner surface of the tube faceplate.
One type of color picture tube has a tension shadow mask mounted within a faceplate panel thereof. The tension shadow mask includes an active apertured portion that contains a plurality of parallel vertically extending strands. A multiplicity of elongated apertures are located between the strands. The electron beams pass through the elongated apertures in the active portion during tube operation.
Handling of tension shadow masks during their manufacture can be very difficult, especially if there are no tie bars or other connections between the strands of the mask. For example, if a tension shadow mask were etched by the process used to make conventional domed masks, there would be excessive movement of the strands during etching, and therefore great difficulty in obtaining repeatable results. The present invention provides a method that utilizes a magnetic assembly to overcome the difficulties that may arise during etching of tension shadow masks.
The present invention provides an improvement in a method of etching apertures in a thin metal sheet to form a shadow mask for a color picture tube. The metal sheet has a first acid-resistant stencil on one major surface thereof and a second acid-resistant stencil on the other major surface thereof. At least one of the stencils has openings therein at locations of intended apertures. The improvement comprises the steps in the etch method of magnetically holding the metal sheet with a flat magnetic assembly, and moving the magnetic assembly magnetically holding the metal sheet thereon through an etching chamber. The magnetic assembly includes a magnetic layer that is supported on an acid-resistant board.